Zirconium and hafnium have unique performance of high-temperature resistance, radiation resistance, corrosion resistance. In the nuclear industry, zirconium alloy is used as reactor cladding and structural material, and hafnium is used as the control material of reactor. Zirconium and hafnium are also widely used in the fields of chemical industry, metallurgy, electronics, etc.
Zirconium and hafnium have very similar chemical properties, and usually mutualistic symbiosis in the ores in the form of oxide. In natural zirconium resources, generally the mass fraction of hafnium is 1.5% to 3% of the zirconium; and in contrast, the nuclear grade sponge zirconium requires w(Hf)<0.01%. Therefore, zirconium and hafnium separation technology is the key for the production of nuclear grade sponge zirconium. In many countries, the method of separating zirconium, hafnium has been studied. Currently, these methods are roughly divided into two categories: hydrometallurgical separation and pyrometallurgical separation.
The hydrometallurgical separation mainly includes MIBK-HCL method, TOA method, TBP-HCL-HNO3 method, improved N235-H2SO4 method and sulfoxides extraction method, etc.
The main principle of pyrometallurgical separation is to separate zirconium and hafnium in the rectifying tower by using the difference of saturated vapor pressure of HfCl4 and ZrCl4 in the molten KAlCl4, and finally to get 30%˜50% enrichment of w(HfCl4) and atomic level of ZrCl4.
The company that uses this technology is COMPAGNIE EUROPEENNE DU ZIRCONIUM-CEZUS. In its patent US20090117018 (publication date: Feb. 7, 2008), the invention titled Process for the separation and purification of hafnium and zirconium disclosed a similar method. By the carbochlorination of zircon core, the mixture of ZrCl4 and HfCl4 is obtained, and the reaction process is as the following equations (1) and (2):ZrSiO4+4C=ZrC+SiO+3CO  (1)ZrC+2Cl2=ZrCl4+C  (2)
ZrCl4 and HfCl4 enter from the middle of the tower, to separate zirconium, hafnium under the condition of atmospheric pressure and 350° C. of tower temperature. The rectifying tower has a number of tower plates, and each tower plate supports the molten salt layer. ZrCl4 fractions are recovered in the solvent phase at the bottom of the tower, while the HfCl4 enriched residual fractions are generated in the vapor phase. The method has the following characteristics: less consumption of chemical reagents, less waste pollution, short separation process, direct connection with the metal reduction process. Its shortcoming is that the equipment and delivery system are carried out at 350˜500° C., which has high requirement for the equipments, with poor purification and impurity removal, large investment, suitable for large-scale zirconium and hafnium smelter.